1. Field of the Invention
This invention relates generally to antennas, and more specifically relates to antennas and antenna systems that determine azimuth and elevation.
2. Description of the Prior Art
The determination of azimuth and elevation or two-dimensional angle-of-arrival (AOA) is widely used in Electronic Warfare Support Measures (ESM) and ELectronic INTelligence (ELINT) radar systems. Interferometry typically provides the basis for making these determinations, and can be performed with linear arrays or circular arrays of antenna elements. Wide-band, high-accuracy, AOA measurement over a full 360-degree instantaneous field-of-view (IFOV) has been identified by the Department of Defense as a required ESM/ELINT capability for battlefield surveillance and targeting systems.
A conventional linear array of antenna elements or linear interferometer 10 is illustrated in FIG. 1. The linear interferometer 10 includes antenna elements 12, and a phase receiver 14. The antenna elements 12 are offset by a predetermined distance d. The linear interferometer 10 uses the difference in phase .phi. between the input signals 16 received by the offset antenna elements 12 to determine the AOA in a planar coordinate system. The sine of the AOA .theta. of the input signals 16 is proportional to the phase difference .phi. between the input signals received by the offset antenna elements 12 in accordance with equation (1) as follows: EQU Phase difference=2.pi.d/.lambda.sin.theta. (1)
In practice, linear interferometers typically utilize three or four antenna elements and provide instantaneous coverage over a 90-120-degree angular sector. Utilizing time-division multiplexers (not shown) to share receiver channels, three or four linear interferometers, each oriented to cover a different 120 degree sector, can provide a 360-degree field-of-view with a three or four-channel phase receiver (not shown). However, since only one linear interferometer is connected to the phase receiver at a given instant in time, this configuration provides only a 90-120-degree IFOV. To obtain a 360-degree IFOV, each antenna element of the three linear interferometers must be connected directly to a channel of the phase receiver. Therefore, nine or twelve or up to sixteen receiver channels are required. Various configurations for sharing a limited number of receiver channels using a greater number of antennas have been devised. These all result in complex, fast switched, receiving systems, or sector-by-sector, time-sequential attachment of the receive system to the antenna.
A conventional circular array of antenna elements or Circular Array Interferometer (CAI) 18 is illustrated in FIG. 2. The CAI 18 includes a plurality of antenna elements 12 arranged in a substantially circular orientation. The antenna elements 12 may be dipoles, monopoles, slots, microstrip patches or alternative types of radiating or receiving antenna elements.
The CAI 18 inherently provides instantaneous coverage over a full 360-degree IFOV. The number of antenna elements is chosen based on AOA accuracy requirements and physical size constraints, but is usually a power of two. Typical CAI's utilize 4, 8, 16 or more antenna elements 12. Each antenna element 12 in the circular array 18 is connected to a Butler matrix input port 20 as shown in the block diagram of FIG. 2. Outputs 22 of the Butler matrix provide omnidirectional phase modes. These are reception responses which are equal magnitude for all AOA, but with phase which is proportional to the value of the AOA. The proportionality constant is an integer equal to the order of the mode. The number of omnidirectional phase modes required for AOA determination in a planar coordinate system is approximately equal to the base 2 logarithm of the number of antenna elements, e.g., 3 modes for 8 elements. Therefore, an eight element CAI needs only three receiver channels. This can be compared to the nine or twelve channels required for the 360-degree IFOV linear interferometer system described above, which requires receiver channels equal to the number of antenna elements. This translates into a smaller, less complex, lower cost, 360-degree IFOV ESM/ELINT system.
The antenna elements 12 of the CAI 18 are located at equiangular distances around the circumference of a circle. While each antenna element 12 typically has only a 90- to 120-degree IFOV, the circular geometry of the CAI 18 inherently provides a full 360-degree IFOV.
A block diagram of an eight-input/eight-output or 8.times.8 Butler matrix 24, utilizing 3-dB quadrature couplers, is given in FIG. 3. The Butler matrix 24 is a network which performs a discrete Fourier transform on a set of the antenna element signals at its inputs using analog signal processing in real-time. The results are the modal outputs of the CAI. The Butler matrix 24 is implemented as a set of 3-dB directional couplers 26 (either quadrature or 0/180 hybrids) and phase shift networks 28 that are interconnected by a labyrinth of transmission lines. While in this example the total number of modal outputs 22 is equal to the number of antenna element inputs 20, only three of the modal outputs 22 are required to accurately determine the AOA of incoming signals. Therefore, only three channels of the phase receiver are required for 360-degree IFOV AOA determination.
Butler matrices are typically passive and reciprocal microwave devices. The manner in which a Butler matrix connected to a circular array establishes phase modes is most easily seen by invoking reciprocity and considering the CAI 18 and Butler matrix 20 as a transmitting configuration. A signal into any mode port 22 of the Butler matrix 24 generally results in signals of equal amplitude and a linear phase gradient at the antenna element ports 20. The phase gradient is determined by which mode port 22 is excited. Exciting a single mode port 22 results in a specific far field radiation or mode pattern. The antenna pattern will have an omnidirectional amplitude and a phase gradient with azimuth angle which matches the phase gradient along the antenna elements of the circular array. A description of the typical Butler matrix, beam forming network and steering circuit (including phase shifters) is provided in U.S. Pat. No. 4,414,550, which is hereby incorporated by reference in its entirety.
Electronically steerable circular radar and communication system arrays are well known in the art and are described in numerous patents, such as, for example, U.S. Pat. No. 4,414,550 to Carl P. Tresselt and U.S. Pat. No. 4,316,192 to Joseph H. Acoraci, which are hereby incorporated by reference in their entirety. Such circular arrays have N number of antenna elements, and are usually coupled to an N.times.N Butler matrix, N or N-1 phase shifters and a signal combining network. As is well known, the transformed signals from the Butler matrix include amplitudes, which are substantially independent of the direction of wavefront incidence, and phase values, which are approximately linearly dependent on direction of wavefront incidence. In these respects, the transformed signals resemble those produced by a linear array. Thus, it is said in the art that the Butler matrix "linearizes" the circular array. Indeed, the plurality of phase shifters, which are used to steer the antenna beam pattern, and the signal combining network, which is used to form the beam patterns of the circular array, are interconnected and controlled in a manner similar to that of the linear array.
For applications where high gain, sensitivity, and selectivity are required, an enhanced CAI system with a unique high gain, narrow beamwidth operating mode is required. Such a system can operate in both a wide open full 360-degree IFOV omnidirectional mode and a narrower IFOV high-gain directional-beam mode capable of full 360-degree coverage via electronic scanning in a planar coordinate system. The block diagram of a 16-element implementation of this system is given in FIG. 4. Low noise preamplifiers 30 are located at the output of each antenna element 12 to mask the losses of the Butler matrix 24 and to maximize sensitivity. The modal outputs of the Butler matrix 24 are connected to phase compensation networks 31 and digital phase shifters 32. The phase shifter outputs are combined to form high gain mode monopulse (sum and difference) directional beams 34. Beam steering is accomplished by appropriate control of the phase shifter 32 settings. The omnidirectional mode is obtained using directional couplers 36 to couple off selected modal outputs 22 of the Butler matrix 24. Therefore, both operating modes are available simultaneously and can be independently controlled.
The conventional approach to obtain highly accurate two dimensional (azimuth and elevation) instantaneous direction finding (DF) over a full hemisphere and the 2-18 GHz frequency band, involves the use of dual, orthogonal linear interferometers in each quadrant, forming the linear interferometer set 38 illustrated in FIG. 5. The antenna elements 12 of the quadrant linear interferometer set 38 are also shown in FIG. 5. Full frequency band coverage is obtained by using two nested sets of antenna elements, one set of antenna elements for 2-6 GHz (the outer, larger set), and another set of antenna elements for 6-18 GHz (the inner, smaller set). A long baseline (separation between the outer elements in each linear array) is required for high accuracy, but this results in AOA ambiguities. Intermediate elements in each array are used to resolve the ambiguities. In the example shown, five elements are used to form one linear interferometer. The orthogonal interferometer also uses five elements but shares one common element, so that the dual pair requires 9 elements per band, per quadrant. Unfortunately, the entire quadrant linear interferometer set requires 72 antenna elements and a like number of receiver channels, while occupying a relatively large volume of approximately 216 cubic feet. In addition, the quadrant linear interferometer 38 is subject to coning errors and its output must be corrected accordingly.
Coning relates to the locus of points on a direction sphere corresponding to constant phase difference between a pair of signals received by a pair of linear array elements. For example, consider the locus for a pair of elements in the horizontally oriented array portion of the quadrant linear interferometer 38. This locus defines a circle or the base of a cone having an axis which is coincident with an axis of the horizontally-oriented array portion. So long as elevation is approximately zero (i.e., at the horizon), the phase difference provides an accurate representation of the azimuth. However, as is well known in the art, for a horizontally-oriented linear interferometer to give an accurate AOA reading off-horizon (non-zero elevation), the elevation measurement must be used to correct the phase difference. This correction is referred to as the coning correction. Phase difference measurements determined by CAI's do not exhibit coning errors and, therefore, do not require coning correction.
FIG. 6 illustrates a stereographic projection of a direction hemisphere. The projection also contains azimuth constant-phase-difference contours 40 and elevation constant-phase-difference contours 42 for the quadrant linear interferometer. An additional disadvantage of using linear interferometers for both azimuth and elevation determinations is that the azimuth constant-phase-difference contours 40 and elevation constant-phase-difference contours 42 are only orthogonal at the horizon 44 as illustrated in FIG. 6. Lack of orthogonality is detrimental because it increases the AOA measurement sensitivity to phase measurement errors. Furthermore, the azimuth constant-phase-difference contours 40 do not align with the radial contours of constant azimuth. Thus, processing (coning correction) is required to relate phase values to corresponding azimuth values.
High accuracy with a linear interferometer requires a large separation (baseline) between array elements. The azimuth angle determined by a long-baseline linear interferometer is ambiguous and needs to be resolved by a supplementary coarse AOA using a shorter baseline determination; this correction is complex. Similarly, with a CAI, high accuracy requires a large diameter circle with many elements equispaced along the circumference. With the prior art, such a CAI has required a high order Butler matrix (such as a 64.times.64 Butler matrix). Such a Butler matrix is both complex and costly.